The invention relates to an apparatus and method for measuring the flow rate and density of moving fluid streams. More specifically, the apparatus and method is designed to measure the force of a moving fluid stream, and to independently measure the velocity of the stream. From these values, the flow rate and density can be derived.
Many operations in the chemical industry, in the recovery of oil and gas from earth formations, and in other industrial processes, require measuring the flow rate and/or the density of a moving fluid stream. The usual fluid streams are liquids, gases, and slurries, or liquids and gases which contain solid materials. There are several different types of conventional instruments now available for measuring flow rates and densities of fluid streams.
Flow meters are generally classified as the "intrusive" type or "non-instrusive" type. In the operation of intrusive flow meters, certain components of the instrument are placed directly in the moving fluid stream, so that this part of the instrument is subjected to the erosive effect of the stream at all times. Conversely, the non-intrusive flow meters are instruments in which the flow measuring components are not inserted directly into the moving stream, so that the erosive effects of the stream are minimized.
A turbine flow meter is an example of a conventional intrusive-type flow meter. The turbine structure is an axial mounted rotor. The rotor is placed inside the pipeline which carries the fluid stream to be measured, so that the stream must pass directly through the rotor. This causes the rotor to turn and the rotor blades induce an AC current in the coil surrounding a permanent magnet in the pickup. The frequency of the AC signal is directly proportional to the liquid velocity. With proper calibration, the output frequency can be used for direct flow rate indication.
The instrument described above, as well as many other commercially available instruments, have certain drawbacks which make them impractical for measuring flow rates of moving fluids in many commercial operations. For example, in some oil well servicing operations, viscous liquids or slurry compositions are injected into the wellbore under high pressure and high velocity. The use of turbine flow meters in these operations has been generally unsatisfactory. For example, the rotor bearings often fail; but a more serious problem is that the space between the rotor and the housing becomes clogged. When this happens, the entire rotor structure can either break loose from the housing, and be carried into the wellbore by the moving fluid, or it becomes seized in the housing and the moving slurry "worm-holes" the housing wall until a serious failure occurs.
The apparatus of this invention has a distinct advantage over the prior devices. For example, none of the parts of this instrument are inserted into the moving fluid stream which is being measured for flow rate, so that this instrument is a non-intrusive flow meter. In addition, the present apparatus is also capable of measuring the density of moving fluid streams, so that it can function both as a flow meter and a densitometer.